Auckland couple Ian and Linda Williams thought they had made an informed decision against immunising their three children because of concerns over adverse reactions.

But they regretted their decision when middle child Alijah contracted the potentially fatal disease just before Christmas, and was put in an induced coma on life support at Starship hospital.

They immediately immunised their other children and wrote to Alijah's school to warn parents who had not vaccinated against the disease and others such as whooping cough.

"It was me that put my son in this situation," Mr Williams said.

"Parents like us make the decision to not vaccinate on very little factual information about the actual consequences of the diseases - massive pain, disability and death - and a lot of non-factual, emotive information from the internet stating inflated figures on the frequency and severity of adverse reactions and conspiracy theories about 'evil' doctors, governments and drug companies."